This couple, who could be described as "middle brown" had twins who are very different in skin colour! Creation Ministries International explains some of the reasons for this in the article The genetics of skin colour in people.
Indeed, the same main substance—a dark brown pigment called melanin—colours the skin of all humans.3 Individuals that have a lot of it appear to have ‘black’ skin; people that have little appear ‘white’. Melanin is produced by organelles called melanosomes, contained in special cells called melanocytes. Melanocytes are located in the bottom layer of the skin, and produce melanin with the aid of tyrosinase and other enzymes. People with lighter-coloured skin have smaller, fewer and less dense melanosomes than darker-skinned people.I've also read that there can be greater genetic variance between two people with white skin than between a white and a black person. The gospel is God's answer to racism, so we should never use skin colour as an excuse not to extend love and hospitality to others. It is awful that people have been segregated, enslaved, poorly treated, and even killed on the basis of skin colour. Christians need to live out the opposite spirit of love, acceptance, and truth. As Christians, our love for people who are different to us is one way we can represent Jesus in this world of racial division and hostility.